11.1.2.3. Extreme Events and Severe Weather Systems
Apart from intraseasonal and interannual variability in climate, extreme weather
events such as heat waves associated with extreme temperatures, extratropical
and tropical cyclones, prolonged dry spells, intense rainfall, tornadoes, snow
avalanches, thunderstorms, and dust storms are known to cause adverse effects
in widely separated areas of Asia. There is some evidence of increases in the
intensity or frequency of some of these extreme weather events on regional scales
throughout the 20th century, although data analyses are relatively poor and
not comprehensive (Balling and Idso, 1990; Bouchard, 1990; Agee, 1991; Yu and
Neil, 1991; Chen et al., 1992; Ostby, 1993; Bardin, 1994; Born, 1996). For example,
increases in climate extremes in the western Siberia-Baikal region and eastern
parts of boreal Asia have been reported in recent decades (Gruza and Rankova,
1997; Gruza et al., 1999). There also are reports of an increase in thunderstorms
over the land regions of tropical Asia (Karl et al., 1995). The frequency and
severity of wildfires in grasslands and rangelands in arid and semi-arid Asia
have increased in recent decades (Pilifosova et al., 1996).
Some mountains in Asia have permanent glaciers that have vacated large areas
during the past few decades, resulting in increases in glacial runoff. As a
consequence, an increased frequency of events such as mudflows and avalanches
affecting human settlements has occurred (Rai, 1999). As mountain glaciers continue
to disappear, the volume of summer runoff eventually will be reduced as a result
of loss of ice resources. Consequences for downstream agriculture, which relies
on this water for irrigation, will be unfavorable in some places. For example,
low- and mid-lying parts of central Asia are likely to change gradually into
more arid, interior deserts.
Countries in temperate Asia have been frequented by many droughts in the 20th
century. In China, droughts in 1972, 1978, and 1997 have been recorded as the
most serious and extensive. A large number of severe floods also have occurred
in China, predominately over the middle and lower basins of the Yangtze (Changjiang),
Huanghe, Huaihe, and Haihe Rivers (Ji et al., 1993). Severe flooding with daily
rainfall exceeding 25 cm struck during July and August 1998 in Korea. In Japan,
drought disasters are significantly more frequent during years following ENSO
warm events than in normal years.
Floods, droughts, and cyclones are the key natural disasters in tropical Asia.
The average annual flood covers vast areas throughout the region: In Bangladesh,
floods cover 3.1 Mha; the total flood-prone area in India is about 40 Mha (Mirza
and Ericksen, 1996). In India, chronically drought-affected areas cover the
western parts of Rajasthan and the Kutch region of Gujarat. However, drought
conditions also have been reported in Bihar and Orissa States in India. In Bangladesh,
about 2.7 Mha are vulnerable to drought annually; there is about 10% probability
that 41-50% of the country is experiencing drought in a given year (Mirza,
1998). Drought or near-drought conditions also occur in parts of Nepal, Papua
New Guinea, and Indonesia, especially during El Niño years. In India,
Laos, the Philippines, and Vietnam, drought disasters are more frequent during
years following ENSO events. At least half of the severe failures of the Indian
summer monsoon since 1871 have occurred during El Niño years (Webster
et al. 1998). In the event of enhanced anomalous warming of the eastern equatorial
Pacific Ocean, such as that observed during the 1998 El Niño, a higher
frequency of intense extreme events all across Asia is possible.
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